Introduction to PHP and DTrace

DTrace is an always-available, low overhead, tracing framework
available on a number of platforms including Solaris, Mac OS X,
Oracle Linux and BSD. DTrace can trace operating system behavior
and user program execution. It can display argument values and be
used to infer performance statistics. Probes are monitored by user
created scripts written in the DTrace D scripting language. This
allows efficient analysis of data points.

PHP probes that are not being actively monitored by a user's DTrace
D script do not contain instrumented code so there is no
performance degradation during normal application execution.
Probes that are being monitored incur an overhead low enough to
generally allow DTrace monitoring on live production systems.

PHP incorporates "User-level Statically Defined Tracing" (USDT)
probes that are triggered at runtime. For example, when a D script
is monitoring PHP's function-entry probe, then,
every time a PHP script function is called, this probe is fired and
the associated D script action code is executed. This action code
could, for example, print probe arguments such as the source file
location of the PHP function. Or the action could aggregate data
such as the number of times each function is called.

Only the PHP USDT probes are described here. Refer to external
general and operating system-specific DTrace literature to see how
DTrace can be used to trace arbitrary functions, and how it can be
used to trace operating system behavior. Note not all DTrace
features are available in all DTrace implementations.

DTrace static probes are included in PHP 5.4. Prior to this they
were available via a » PECL extension, which is now
obsolete.

The static DTrace probes in PHP can alternatively be used with the
SystemTap facility on some Linux distributions.